Modern microwave and millimeter wave technology requires solid state sources, mixers, and amplifiers which can operate at extremely high frequency, which have low-noise characteristics, and which can be integrated in an IC (integrated circuit) technology. Devices for the highest frequencies generally must be made from GaAs, or other III-V IC's, which are much more expensive than silicon IC's. Alternatively, the III-V device can be hybridized with silicon IC's, which introduces difficult and expensive processing steps.
A resonant tunnel diode (RTD), made from either silicon or a III-V semiconductor, has a Negative Differential Resistance (NDR) in its I-V characteristic and as a result has the capability of operation as an oscillator, an amplifier, or a mixer at extremely high frequencies with reasonable current density and with very low noise. Such RTD's must be fabricated with an epitaxial growth technique such as MBE, MOCVD, MOMBE, etc. Devices made using these techniques will be expensive and difficult to integrate with a less-expensive IC technology.